Electricity: measuring and testing – Of geophysical surface or subsurface in situ – For small object detection or location
Patent
1997-08-25
1999-06-29
Strecker, Gerard
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
For small object detection or location
175 45, 32420726, 324244, G01V 308, G01V 3165
Patent
active
059173254
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for locating inaccessible objects, particularly, but not exclusively, for locating underground boring tools.
2. Summary of the Prior Art
When locating underground objects it is known to arrange for a magnetic field to be generated by the object or by a field source attached thereto. In the case of underground conductors such as cables or pipes, an alternating current can be applied to the conductor to induce a cylindrical magnetic field with a detectable field strength at ground level. By measuring the variation of the field strength at the surface, the path of the underground conductor can be determined.
The locating of objects such as sub-surface boring tools cannot normally be performed in this way because a cylindrical field cannot be generated. Instead a magnetic field source is mounted on the boring tool and the field from that field source is detected. This field source can be a solenoid. When alternating current flows through the solenoid a bipolar magnetic field is generated which can be located at the surface by a person with a hand-held detector. The vertical component of the field at the surface will change direction when the field detector is directly above the solenoid. Therefore by noting the position in which that component of the field reverses the position of the solenoid in a horizontal plane can be determined. If this is done continuously, the movement of the boring tool on which the solenoid is mounted can be tracked. The depth of the solenoid can also be gauged by measuring the attenuation of the field at the surface. Of course, this requires the field strength at the solenoid to be known.
The method described above for locating a solenoid is a method of trial-and-error. The field detector must be maneuvered into the correct position, namely directly above the solenoid, for the solenoid to be accurately located. When the field detector is anywhere other than directly above the solenoid, the location of the solenoid is not known. This is inconvenient, especially if the solenoid is underneath a road or a waterway.
SUMMARY OF THE PRESENT INVENTION
The present invention proposes a method of locating a solenoid carrying an alternating electric current by detecting the magnetic field generated by the solenoid at a remote site, so that the need for detecting the field from directly above the solenoid is eliminated. The solenoid may be mounted on a sub-surface boring tool, which can then be controlled as required according to its measured location.
The present invention makes use of the realization that there is a known relationship between the ratio of the axial and radial magnetic fields at any point in the plane containing the axis of the solenoid and the angle between the axis of the solenoid and the line joining the point to the center of the solenoid. Therefore, the present invention proposes that information corresponding to that relationship be stored, e.g. in a computer, so that measurement of the axial and radial fields enable the angle to be determined, thereby to enable the direction from the measuring point to the solenoid to be determined.
Since the invention must determine the axial and radial fields, to enable the angle to be determined, it is necessary to know the orientation of the solenoid. If it is possible to maintain the solenoid in a predetermined orientation, then there is no difficulty. However, if the solenoid is mounted in an object which moves underground, such as a boring tool, its orientation may change. To compensate for this the boring tool may then incorporate a tilt sensor which permits the orientation of the solenoid relative to the horizontal to be determined, thus determining the direction of the axial and radial fields that must be determined. Normally, those axial and radial fields will be measured directly, but it is possible to measure e.g. horizontal and vertical components, and then calculate the axial and radial fields.
Preferably, the absolute value
REFERENCES:
patent: 4710708 (1987-12-01), Rorden et al.
patent: 4875014 (1989-10-01), Roberts et al.
patent: 5155442 (1992-10-01), Mercer
patent: 5621325 (1997-04-01), Draper et al.
patent: 5633589 (1997-05-01), Mercer
Radiodetection Limited
Strecker Gerard
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